A US army psychiatrist accused of killing 13 people at a Texas military base said Tuesday he plans to mount a defense arguing he attacked soldiers in order to defend the Taliban.

Major Nidal Hasan, who faces the death penalty if convicted, has asked to delay his upcoming court martial so he can prepare to represent himself.

Hasan, 42, is also accused of wounding 32 other people in the 2009 Fort Hood massacre -- people who could now be called as witnesses he would then be allowed to question in court.

Judge Colonel Tara Osborn on Tuesday agreed to postpone jury selection, which was set to begin Wednesday, while she considers Hasan's request for a three month postponement.

Hasan told the judge he plans to mount a "defense of others" argument, which can be used in the same way as a self-defense argument in cases where other people are in danger of imminent harm.

"They were in the US Army," Hasan said of his victims. "They were... about to deploy to Afghanistan."

Hasan said he mounted the attack to "protect the leaders of the Islamic Emirate of Afghanistan, the Taliban - Mullah Omar is their leader."

A former lawyer with the base's judge advocate general's office called the tactic "frivolous" and said the strategy is flawed because the victims were not in the process of attacking anyone.

"I don't see how that works here. I don't think it applies," Richard Rosen, a law professor at Texas Tech University, told AFP.

"If you make a mistake as to who the aggressor is, you pay the price."

The shooting at Fort Hood jolted the US military and prompted calls for stronger safeguards against possible internal security threats and "homegrown" terror attacks.

The FBI alleges Hasan had contacts with firebrand US-born cleric Anwar al-Awlaqi, a key leader of Al-Qaeda in the Arabian Peninsula who was killed in a 2011 drone strike.

Hasan, who was born in Virginia to Palestinian parents and raised in the eastern US state, had attended a mosque in 2001 where Awlaqi worked and is believed to have continued to communicate with the cleric.

Hasan has repeatedly attempted to plead guilty but the request was denied because prosecutors were unwilling to waive the death penalty, and military law does not permit people to plead guilty to a capital offense.

The shooting left Hasan paralyzed from the chest down after civilian police at the base shot him multiple times.